Method and system for sealing a substrate

ABSTRACT

A method of sealing a microelectromechanical system (MEMS) device from ambient conditions is described. The MEMS device is formed on a substrate and a substantially hermetic seal is formed as part of the MEMS device manufacturing process. The method may include forming a metal seal on the substrate proximate to a perimeter of the MEMS device using a method such as photolithography. The metal seal is formed on the substrate while the MEMS device retains a sacrificial layer between conductive members of MEMS elements, and the sacrificial layer is removed after formation of the seal and prior to attachment of a backplane.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/842,916, filed Aug. 21, 2007, now issued U.S. Pat. No. 7,629,678,which is a continuation of U.S. application Ser. No. 11/089,769, filedon Mar. 16, 2005, now U.S. Pat. No. 7,259,449, which claims priority toU.S. Provisional Patent Application No. 60/613,569, filed on Sep. 27,2004. The disclosure of each of the above-described filed applicationsis hereby incorporated by reference in its entirety.

BACKGROUND

1. Field of the Invention

The field of the invention relates to microelectromechanical systems(MEMS), and more particularly to a method of sealing a MEMS device fromambient conditions.

2. Description of the Related Art

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. An interferometricmodulator may comprise a pair of conductive plates, one or both of whichmay be transparent and/or reflective in whole or part and capable ofrelative motion upon application of an appropriate electrical signal.One plate may comprise a stationary layer deposited on a substrate, theother plate may comprise a metallic membrane separated from thestationary layer by an air gap. Such devices have a wide range ofapplications, and it would be beneficial in the art to utilize and/ormodify the characteristics of these types of devices so that theirfeatures can be exploited in improving existing products and creatingnew products that have not yet been developed.

SUMMARY

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Certain Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

In one aspect, an electronic device including a micro-electromechanicalsystems (MEMS) device, a metal layer, a mask, a metal seal layer, and abackplane is prepared by a process comprising the steps of providing aMEMS device on a substrate; depositing a metal layer on the substrate;forming a mask with one or more perimeter cavities over the metal layer;depositing one or more metal seal layers in the one or more perimetercavities, thereby forming a seal proximate to the perimeter of the MEMSdevice; and joining a backplane to the seal.

In some embodiments, the MEMS device comprises an interferometricmodulator. In some embodiments, the MEMS device is further prepared by aprocess comprising removing the mask, and the metal layer, at areas notcovered by the one or more metal seal layers, thereby forming a sealantwall. In some embodiments, the MEMS device is further prepared by aprocess comprising removing a sacrificial layer from the MEMS device. Insome embodiments, the MEMS device is further prepared by a processcomprising applying a desiccant between the MEMS device and thebackplane. In some embodiments, the desiccant is selected from the groupconsisting of zeolites, molecular sieves, surface adsorbents, bulkadsorbents and chemical reactants. In some embodiments, the desiccantcomprises a powder. In some embodiments, the seal proximate to theperimeter of the MEMS device comprises a non-hermetic seal. In someembodiments, the non-hermetic seal is selected from the group consistingof conventional epoxy-based adhesive, polyisobutylene, butyl rubber,o-rings, polyurethane, thin film metal weld, liquid spin-on glass,solder, polymers and plastic. In some embodiments, the seal proximate tothe perimeter of the MEMS device comprises a substantially hermeticseal. In some embodiments, the substantially hermetic seal comprisesmetal.

In some embodiments, the MEMS device is further prepared by depositingan insulator layer on the substrate before depositing the metal layer.In some embodiments, the depositing the insulator layer comprisesdepositing the insulator layer to contact the substrate. In someembodiments, the depositing the insulator layer comprises depositing theinsulator layer on at least one layer in contact with the substrate. Insome embodiments, the at least one layer comprises a conductivematerial.

In another aspect, a method of sealing a microelectromechanical system(MEMS) device from ambient conditions comprises forming a substantiallymetal seal on a substrate comprising a MEMS device, and attaching abackplane to the metal seal so as to seal the MEMS device from ambientconditions.

Forming the substantially metal seal may comprise forming an insulatorlayer on the substrate, and forming a metal sealant wall on theinsulator layer, and the method may further comprise forming an adhesivelayer on the metal seal for attachment of the backplane. In someembodiments, attaching the backplane comprises soldering.

Another aspect of a method of sealing a MEMS device from ambientconditions comprises forming a MEMS device on a substrate, wherein theMEMS device comprises a sacrificial layer, depositing an insulator layerover the MEMS device and the substrate, depositing one or more metallayers over the insulator layer, and forming a mask with one or morecavities over the one or more metal layers, wherein the cavities definea perimeter around the MEMS device. The method further comprises formingone or more metal seal layers in the one or more cavities in the mask,thereby forming a substantially hermetic seal proximate to the perimeterof the MEMS device, removing the mask layer, the one or more metallayers, and the insulating layer to form a sealant wall around theperimeter of the MEMS device, removing the sacrificial layer from theMEMS device, and attaching a backplane to the sealant wall to seal theMEMS device from ambient conditions.

In some embodiments, forming the one or more metal seal layers compriseselectroplating over the mask layer. In certain embodiments, the maskcomprises photoresist, wherein forming the mask comprises the use of UVlight.

The method may further comprise forming one or more adhesion metallayers over the one or more metal seal layers, wherein the one or moreadhesion metal layers are configured for attachment to the backplane.The one or more adhesion layers may comprise a solder, for example.

In some embodiments, the one or metal layers deposited over theinsulator layer comprise at least one of a metal seed layer and anadhesion layer. In some embodiments, the backplane comprises apre-deposited adhesion layer configured to adhere to the sealant wall.

Attaching the backplane to the sealant wall may comprise soldering, andthe backplane may comprise an adhesion layer and a solder layerproximate to an area for attachment to the sealant wall, wherein theadhesion layer comprises metal, for example.

Yet another aspect of a method of packaging a MEMS device comprisesdepositing an insulator over a MEMS device formed on a substrate,wherein the MEMS device includes a sacrificial layer, depositing one ormore metal layers over the insulator, and forming a mask with one ormore cavities over the metal layer. The method further comprises formingone or more metal seal layers in the one or more cavities, therebyforming a substantially hermetic seal proximate to a perimeter of theMEMS device, removing the mask layer, the one or more metal layers, andthe insulating layer, removing the sacrificial layer from the MEMSdevice, and positioning a backplane in contact with the seal so as toseal the MEMS device from ambient conditions.

In some embodiments, forming one or more metal seal layers compriseselectroplating over the mask layer, and the mask may comprisephotoresist, wherein forming the mask comprises the use of UV light.

The method may further comprise forming one or more adhesion metallayers over the one or more metal seal layers, wherein the one or moreadhesion metal layers are configured for attachment to the backplane,and the one or more adhesion layers may comprise a solder, for example.

The one or metal layers deposited over the insulator layer may compriseat least one of a metal seed layer and an adhesion layer, and thebackplane may comprise a pre-deposited adhesion layer configured toadhere to the sealant wall.

In some embodiments, attaching the backplane to the sealant wallcomprises soldering. The backplane may comprise an adhesion layer and asolder layer proximate to an area for attachment to the sealant wall,and the adhesion layer may comprise metal, for example.

In another aspect, a system for sealing a MEMS device from ambientconditions comprises a MEMS device formed on a substrate, means forproviding a substantially metal seal on the substrate and proximate to aperimeter of the MEMS device, thereby forming a substantially hermeticseal proximate to a perimeter of the MEMS device, and a backplane incontact with the substantially metal seal, thereby encapsulating theMEMS device within the substrate, the substantially metal seal, and thebackplane.

The system may further comprise means for forming one or more adhesionmetal layers over the one or more metal seal layers, wherein the one ormore adhesion metal layers are configured for attachment to a backplane.The one or more adhesion layers may comprise a solder, for example. Inaddition, the metal seal may comprise at least one of a metal seed layerand an adhesion layer.

In certain embodiments, the system further comprises means for attachingthe backplane to the metal seal. The means for attaching the backplanemay comprise a pre-deposited adhesion layer configured to adhere to themetal seal. The means for attaching the backplane may comprise a solder.In some embodiments, the means for attaching the backplane comprises anadhesion layer and a solder layer on the backplane proximate to an areafor attachment to the metal seal, and the adhesion layer may comprisemetal, for example.

In another aspect, a MEMS device sealed from ambient conditionscomprises a MEMS device formed on a substrate, a substantially metalseal formed on the substrate proximate to a perimeter of the MEMSdevice, and a backplane in contact with the substantially metal seal,thereby encapsulating the MEMS device within the substrate, thesubstantially metal seal, and the backplane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a released position and amovable reflective layer of a second interferometric modulator is in anactuated position.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIG. 6A is a cross section of the device of FIG. 1.

FIG. 6B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 6C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7A is a cross-sectional view of a basic package structure for aninterferometric modulator device.

FIG. 7B is an isometric view of the package structure of FIG. 7A with ametal seal.

FIGS. 8A-8E are cross-sectional views illustrating progressive stages ofone embodiment of a method of forming a metal seal and packaging a MEMSdevice.

FIG. 9 is a flow chart illustrating one embodiment of a method offorming a plated metal seal for a MEMS packaging structure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One embodiment of the invention is a MEMS based device packagecomprising a MEMS device on a substrate, wherein a seal is positionedproximate to a perimeter of the MEMS device and a backplane is joined tothe seal so as create a cavity to encapsulate the MEMS device. The sealpreferably comprises one or more metal layers insulated from thesubstrate, wherein the seal is configured to prevent moisture fromentering the package.

The following detailed description is directed to certain specificembodiments of the invention. However, the invention can be embodied ina multitude of different ways. In this description, reference is made tothe drawings wherein like parts are designated with like numeralsthroughout. As will be apparent from the following description, theinvention may be implemented in any device that is configured to displayan image, whether in motion (e.g., video) or stationary (e.g., stillimage), and whether textual or pictorial. More particularly, it iscontemplated that the invention may be implemented in or associated witha variety of electronic devices such as, but not limited to, mobiletelephones, wireless devices, personal data assistants (PDAs), hand-heldor portable computers, GPS receivers/navigators, cameras, MP3 players,camcorders, game consoles, wrist watches, clocks, calculators,television monitors, flat panel displays, computer monitors, autodisplays (e.g., odometer display, etc.), cockpit controls and/ordisplays, display of camera views (e.g., display of a rear view camerain a vehicle), electronic photographs, electronic billboards or signs,projectors, architectural structures, packaging, and aestheticstructures (e.g., display of images on a piece of jewelry). MEMS devicesof similar structure to those described herein can also be used innon-display applications such as in electronic switching devices.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“on” or “open”) state, the display element reflects a large portion ofincident visible light to a user. When in the dark (“off” or “closed”)state, the display element reflects little incident visible light to theuser. Depending on the embodiment, the light reflectance properties ofthe “on” and “off” states may be reversed. MEMS pixels can be configuredto reflect predominantly at selected colors, allowing for a colordisplay in addition to black and white.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical cavity with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as thereleased state, the movable layer is positioned at a relatively largedistance from a fixed partially reflective layer. In the secondposition, the movable layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable and highly reflective layer 14 ais illustrated in a released position at a predetermined distance from afixed partially reflective layer 16 a. In the interferometric modulator12 b on the right, the movable highly reflective layer 14 b isillustrated in an actuated position adjacent to the fixed partiallyreflective layer 16 b.

The fixed layers 16 a, 16 b are electrically conductive, partiallytransparent and partially reflective, and may be fabricated, forexample, by depositing one or more layers each of chromium andindium-tin-oxide onto a transparent substrate 20. The layers arepatterned into parallel strips, and may form row electrodes in a displaydevice as described further below. The movable layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes 16 a, 16 b) deposited on top ofposts 18 and an intervening sacrificial material deposited between theposts 18. When the sacrificial material is etched away, the deformablemetal layers are separated from the fixed metal layers by a defined airgap 19. A highly conductive and reflective material such as aluminum maybe used for the deformable layers, and these strips may form columnelectrodes in a display device.

With no applied voltage, the cavity 19 remains between the layers 14 a,16 a and the deformable layer is in a mechanically relaxed state asillustrated by the pixel 12 a in FIG. 1. However, when a potentialdifference is applied to a selected row and column, the capacitor formedat the intersection of the row and column electrodes at thecorresponding pixel becomes charged, and electrostatic forces pull theelectrodes together. If the voltage is high enough, the movable layer isdeformed and is forced against the fixed layer (a dielectric materialwhich is not illustrated in this Figure may be deposited on the fixedlayer to prevent shorting and control the separation distance) asillustrated by the pixel 12 b on the right in FIG. 1. The behavior isthe same regardless of the polarity of the applied potential difference.In this way, row/column actuation that can control the reflective vs.non-reflective pixel states is analogous in many ways to that used inconventional LCD and other display technologies.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application. FIG. 2is a system block diagram illustrating one embodiment of an electronicdevice that may incorporate aspects of the invention. In the exemplaryembodiment, the electronic device includes a processor 21 which may beany general purpose single- or multi-chip microprocessor such as an ARM,Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051,a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessorsuch as a digital signal processor, microcontroller, or a programmablegate array. As is conventional in the art, the processor 21 may beconfigured to execute one or more software modules. In addition toexecuting an operating system, the processor may be configured toexecute one or more software applications, including a web browser, atelephone application, an email program, or any other softwareapplication.

In one embodiment, the processor 21 is also configured to communicatewith an array controller 22. In one embodiment, the array controller 22includes a row driver circuit 24 and a column driver circuit 26 thatprovide signals to a pixel array 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. For MEMSinterferometric modulators, the row/column actuation protocol may takeadvantage of a hysteresis property of these devices illustrated in FIG.3. It may require, for example, a 10 volt potential difference to causea movable layer to deform from the released state to the actuated state.However, when the voltage is reduced from that value, the movable layermaintains its state as the voltage drops back below 10 volts. In theexemplary embodiment of FIG. 3, the movable layer does not releasecompletely until the voltage drops below 2 volts. There is thus a rangeof voltage, about 3 to 7 V in the example illustrated in FIG. 3, wherethere exists a window of applied voltage within which the device isstable in either the released or actuated state. This is referred toherein as the “hysteresis window” or “stability window.” For a displayarray having the hysteresis characteristics of FIG. 3, the row/columnactuation protocol can be designed such that during row strobing, pixelsin the strobed row that are to be actuated are exposed to a voltagedifference of about 10 volts, and pixels that are to be released areexposed to a voltage difference of close to zero volts. After thestrobe, the pixels are exposed to a steady state voltage difference ofabout 5 volts such that they remain in whatever state the row strobe putthem in. After being written, each pixel sees a potential differencewithin the “stability window” of 3-7 volts in this example. This featuremakes the pixel design illustrated in FIG. 1 stable under the sameapplied voltage conditions in either an actuated or releasedpre-existing state. Since each pixel of the interferometric modulator,whether in the actuated or released state, is essentially a capacitorformed by the fixed and moving reflective layers, this stable state canbe held at a voltage within the hysteresis window with almost no powerdissipation. Essentially no current flows into the pixel if the appliedpotential is fixed.

In typical applications, a display frame may be created by asserting theset of column electrodes in accordance with the desired set of actuatedpixels in the first row. A row pulse is then applied to the row 1electrode, actuating the pixels corresponding to the asserted columnlines. The asserted set of column electrodes is then changed tocorrespond to the desired set of actuated pixels in the second row. Apulse is then applied to the row 2 electrode, actuating the appropriatepixels in row 2 in accordance with the asserted column electrodes. Therow 1 pixels are unaffected by the row 2 pulse, and remain in the statethey were set to during the row 1 pulse. This may be repeated for theentire series of rows in a sequential fashion to produce the frame.Generally, the frames are refreshed and/or updated with new display databy continually repeating this process at some desired number of framesper second. A wide variety of protocols for driving row and columnelectrodes of pixel arrays to produce display frames are also well knownand may be used in conjunction with the present invention.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Releasing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias).

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows are at 0volts, and all the columns are at +5 volts. With these applied voltages,all pixels are stable in their existing actuated or released states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and releases the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and release pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. It will be appreciated that the same procedurecan be employed for arrays of dozens or hundreds of rows and columns. Itwill also be appreciated that the timing, sequence, and levels ofvoltages used to perform row and column actuation can be varied widelywithin the general principles outlined above, and the above example isexemplary only, and any actuation voltage method can be used with thepresent invention.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 6A-6C illustrate three different embodiments of themoving mirror structure. FIG. 6A is a cross section of the embodiment ofFIG. 1, where a strip of metal material 14 is deposited on orthogonallyextending supports 18. In FIG. 6B, the moveable reflective material 14is attached to supports at the corners only, on tethers 32. In FIG. 6C,the moveable reflective material 14 is suspended from a deformable layer34. This embodiment has benefits because the structural design andmaterials used for the reflective material 14 can be optimized withrespect to the optical properties, and the structural design andmaterials used for the deformable layer 34 can be optimized with respectto desired mechanical properties. The production of various types ofinterferometric devices is described in a variety of publisheddocuments, including, for example, U.S. Published Application2004/0051929. A wide variety of well known techniques may be used toproduce the above described structures involving a series of materialdeposition, patterning, and etching steps.

The moving parts of a MEMS device, such as an interferometric modulatorarray, preferably have a protected space in which to move. Packagingtechniques for a MEMS device will be described in more detail below. Aschematic of a basic package structure for a MEMS device, such as aninterferometric modulator array, is illustrated in FIG. 7A. As shown inFIG. 7A, a basic package structure 70 includes a substrate 72 and abackplane cover or “cap” 74, wherein an interferometric modulator array76 is formed on the substrate 72. This cap 74 is also called a“backplane”.

The substrate 72 and the backplane 74 are joined by a seal 78 to formthe package structure 70, such that the interferometric modulator array76 is encapsulated by the substrate 72, backplane 74, and the seal 78.This forms a cavity 79 between the backplane 74 and the substrate 72.The seal 78 may be a non-hermetic seal, such as a conventionalepoxy-based adhesive. In other embodiments, the seal 78 may be apolyisobutylene (sometimes called butyl rubber, and other times PIB),o-rings, polyurethane, thin film metal weld, liquid spin-on glass,solder, polymers, or plastics, among other types of seals that may havea range of permeability of water vapor of about 0.2-4.7 g mm/m²kPa day.In still other embodiments, the seal 78 may be a hermetic seal.

In some embodiments, the package structure 70 includes a desiccant 80configured to reduce moisture within the cavity 79. The skilled artisanwill appreciate that a desiccant may not be necessary for a hermeticallysealed package, but may be desirable to control moisture resident withinthe package. In one embodiment, the desiccant 80 is positioned betweenthe interferometric modulator array 76 and the backplane 74. Desiccantsmay be used for packages that have either hermetic or non-hermeticseals. In packages having a hermetic seal, desiccants are typically usedto control moisture resident within the interior of the package. Inpackages having a non-hermetic seal, a desiccant may be used to controlmoisture moving into the package from the environment. Generally, anysubstance that can trap moisture while not interfering with the opticalproperties of the interferometric modulator array may be used as thedesiccant 80. Suitable desiccant materials include, but are not limitedto, zeolites, molecular sieves, surface adsorbents, bulk adsorbents, andchemical reactants.

The desiccant 80 may be in different forms, shapes, and sizes. Inaddition to being in solid form, the desiccant 80 may alternatively bein powder form. These powders may be inserted directly into the packageor they may be mixed with an adhesive for application. In an alternativeembodiment, the desiccant 80 may be formed into different shapes, suchas cylinders or sheets, before being applied inside the package.

The skilled artisan will understand that the desiccant 80 can be appliedin different ways. In one embodiment, the desiccant 80 is deposited aspart of the interferometric modulator array 76. In another embodiment,the desiccant 80 is applied inside the package 70 as a spray or a dipcoat.

The substrate 72 may be a semi-transparent or transparent substancecapable of having thin film, MEMS devices built upon it. Suchtransparent substances include, but are not limited to, glass, plastic,and transparent polymers. The interferometric modulator array 76 maycomprise membrane modulators or modulators of the separable type. Theskilled artisan will appreciate that the backplane 74 may be formed ofany suitable material, such as glass, metal, foil, polymer, plastic,ceramic, or semiconductor materials (e.g., silicon).

The packaging process may be accomplished in a vacuum, pressure betweena vacuum up to and including ambient pressure, or pressure higher thanambient pressure. The packaging process may also be accomplished in anenvironment of varied and controlled high or low pressure during thesealing process. There may be advantages to packaging theinterferometric modulator array 76 in a completely dry environment, butit is not necessary. Similarly, the packaging environment may be of aninert gas at ambient conditions. Packaging at ambient conditions allowsfor a lower cost process and more potential for versatility in equipmentchoice because the device may be transported through ambient conditionswithout affecting the operation of the device.

Generally, it is desirable to minimize the permeation of water vaporinto the package structure and thus control the environment inside thepackage structure 70 and hermetically seal it to ensure that theenvironment remains constant. When the humidity within the packageexceeds a level beyond which surface tension from the moisture becomeshigher than the restoration force of a movable element (not shown) inthe interferometric modulator 10, the movable element may becomepermanently stuck to the surface.

As noted above, a desiccant may be used to control moisture residentwithin the package structure 70. However, the need for a desiccant canbe reduced or eliminated with the implementation of a hermetic seal 78to prevent moisture from traveling from the atmosphere into the interiorof the package structure 70.

The continued reduction in display device dimensions restricts availablemethods to manage the environment within the package structure 70because there is less area to place a desiccant 80 within the packagestructure 70. Although the area of a packaging structure susceptible toinflux of water vapor may remain the same or be slightly reduced aspackage structures are reduced in size, the area available for adesiccant is reduced dramatically in comparison. The elimination of theneed for a desiccant also allows the package structure 70 to be thinner,which is desirable in some embodiments. Typically, in packagescontaining desiccants, the lifetime expectation of the packaged devicemay depend on the lifetime of the desiccant. When the desiccant is fullyconsumed, the interferometric modulator device may fail as sufficientmoisture enters the package structure and damages the interferometricmodulator array.

In one embodiment, the seal 78 comprises a plated metal that forms abarrier configured to act as an environmental barrier inhibiting orpreventing moisture flow therethrough. FIG. 7B is an isometricillustration of a packaging structure 70 with a metal seal 78. In oneembodiment, the seal 78 is a hermetic seal that prevents air and watervapor flow through the seal 78. The metal seal 78 is preferably formedaccording to a photolithographic process and allows for positioning ofthe seal 78 within +/−1 μm of a desired position.

FIGS. 8A-8E are cross-sectional views illustrating different stages ofone embodiment of a method of forming the seal 78 illustrated in FIG.7B, and packaging a MEMS device such as the interferometric modulatorarray 76.

FIG. 8A is a cross-sectional view illustrating the interferometricmodulator array 76 formed on the substrate 72. In one embodiment, theinterferometric modulator array 76 still comprises a sacrificial layer(not shown) between the conductive members, such as the conductivemembers 14, 16 illustrated in FIG. 1, of the interferometric modulatorelements of the array 76. The sacrificial layer may comprise molybdenum,for example. In reference to FIG. 8B, conductive leads 801 on thesubstrate 72 are electrically isolated from the seal by depositing aninsulator 802 over the substrate 72. In certain embodiments, theinsulator 802 is also deposited over the interferometric modulator array76, as illustrated in FIG. 8B. In some embodiments, the insulator 802may be applied to a location on the substrate 72 where the seal 78 is tobe formed. However, it may be more practical to form the insulator 802by deposition on top of the substrate 72 and interferometric modulatorarray 76 as illustrated in FIG. 8B, and subsequently remove undesiredportions of the insulator 802. The insulator can be formed, for example,by thin film deposition methods such as sputtering or chemical vapordeposition (CVD). In one embodiment, the insulator is SiO₂ or otherinsulating oxide or nitride, and has a thickness of about 2000 Å orless, or about 1000 Å or less. As will be appreciated by those skilledin the technology, other methods of deposition of the insulator andalternative insulator materials are within the scope of the invention.

Following deposition of the insulator 802, one or more metal layers 804for electroplating are deposited directly on top of the insulator 802.The metal layer 804 may comprise a plurality of layers, and in oneembodiment, the metal layers 804 include an adhesion layer and a metalseed layer. The adhesion layer preferably promotes adhesion between theinsulator and the metal seed layer. In certain embodiments, the metalseed layer is a conductive plating base upon which electrodeposit isformed and does not need to be the same material as the electrodeposit.In one embodiment, the metal seed layer has a thickness of about500-2000 Å and the adhesion layer has a thickness of about 50-100 Å. Inone embodiment, the combined thickness of the adhesion layer and themetal seed layer is about 100-500 Å. In some embodiments, the adhesionlayer comprises different types of metal, such as titanium (Ti) orchromium (Cr). The skilled artisan will understand that it is alsopossible to dispense with the adhesion layer if the surface to which themetal seed layer is to be attached is rough, for example.

As illustrated in FIG. 8C, a mask 806 is patterned over the metal layers804 to define one or more cavities 808 for the formation of the sealantwall proximate to a perimeter of the interferometric modulator array 76.The perimeter cavity 808 may be square or rectangular, or may have othergeometries. In an embodiment of the package structure 70 containing morethan one interferometric modulator arrays 76, the seal 78 is positionedaround a total perimeter of the plurality of interferometric modulatorarrays. In one embodiment, the metal layers 804 are deposited afterpatterning the mask 806, such that the metal layers 804 are deposited onthe insulator 802 only in the cavities 808.

The mask 806 preferably comprises photoresist and can be patterned usingUV light, for example. The photoresist can comprise an organic polymerthat becomes soluble when exposed to ultraviolet light and preventsetching or plating of the area it covers (this is also known as resist).Photoresist and the use thereof is well known in various industries suchas semiconductor, biomedical engineering, holographic, electronics, andnanofabrication. In certain embodiments, the use of photoresist as themask is preferable so as to define thick layers with precision.

As illustrated in FIG. 8D, one or more layers of metal are depositedonto the exposed metal seed layer 804 in the cavity 808, thereby forminga sealant wall 810. Deposition of the one or more metal layerspreferably comprises electroplating using an electroplating bath. Othermethods of depositing or forming the metal layers are contemplated,including, for example, electroless plating.

In one embodiment, the thickness of the mask 806 is dependent upon thedesired thickness or height of the sealant wall 810, wherein thethickness of the mask 806 is substantially the same or slightly greaterthan the desired height of the sealant wall 810. In certain embodiments,the mask 806 is thicker than the desired height of the sealant wall 810,or less thick than the desired height of the sealant wall 810. In oneembodiment, the height of the sealant wall 810 is about 10-70 μm. Insome embodiments, the height of the sealant wall 810 is about 30-50 μm.Other heights for the sealant wall 810 are contemplated.

As illustrated in FIG. 8E, the mask 806, metal layers 804, and insulator802 are removed at areas not covered by the electroplated metal 808 by amethod such as a wet chemical etch. Alternatively, the metal layers 804and insulator 802 may each be removed in separate steps. In theembodiment wherein the metal seed layer is deposited on top of the mask806, the metal seed layer is removed with the mask 806. Prior to joiningthe backplane 74 to the seal 78, the sacrificial layer present in theinterferometric modulator array 76 is etched, using XeF₂ gas forexample, to release the membranes or conductive members of theinterferometric modulator elements of the array 76.

After the sacrificial layer is removed from the interferometricmodulator array 76, the backplane 74 is joined to the seal 78, whichincludes the insulator 802 and sealant wall 810, to form the packagestructure 70. The skilled artisan will appreciate that the backplane 74may be formed of any suitable material, such as glass, metal, foil,polymer, plastic, ceramic, or semiconductor materials (e.g., silicon).

Referring to FIG. 7A, a person of ordinary skill in the art can selectthe height of the seal 78 and features of the backplane 74 such that thedesiccant 80 is sufficiently distanced from the interferometricmodulator array 76. The height of the seal 78 is directly proportionalto the height of the sealant wall 810. In certain embodiments, the seal78 is preferably formed to a height of about 100-300 μm. In otherembodiments, the seal 78 is preferably formed to a height of less thanabout 400 μm. In other embodiments, the seal 78 is preferably formed toa thickness greater than about 50 μm. The skilled artisan willappreciate that the thickness of the seal 78 may depend on variousfactors, such as the desired lifetime of the interferometric modulatorarray 76, the material of the seal 78, the amount of contaminants andmoisture that are estimated to permeate into the package structure 70during the lifetime of the array 76, the humidity of the ambientenvironment, and/or whether a desiccant 80 is included within thepackage structure 70.

FIG. 9 is a process flow diagram illustrating one embodiment of a methodof forming a metal seal and packaging a MEMS device. In someembodiments, the structure illustrated in FIG. 8A is the beginningstructure for performance of the method 900 of FIG. 9, which begins in astep 902. The method 900 proceeds to a step 902 wherein the seal iselectrically isolated from conductive leads located on the substrate bydepositing an insulator over the substrate and the interferometricmodulator array, as illustrated in FIG. 8B.

Following deposition of the insulator in step 904, one or more metallayers for electroplating are deposited directly on top of the insulatorin a step 906. As discussed above in reference to FIG. 8B, the one ormore metal layers may comprise a plurality of layers, and in oneembodiment, the metal layers include an adhesion layer and a metal seedlayer. In a step 908, a mask is patterned over the metal layers formedin step 906 in order to define one or more cavities 808 for theformation of the sealant wall proximate to a perimeter of theinterferometric modulator array.

In a step 910, electroplating is performed using an electroplating bathto deposit one or more layers of metal onto the exposed metal seed layerin the cavity formed in step 908, thereby forming a sealant wall.Electroplating is capable of depositing over half of the elements of theperiodic table. In one embodiment, the preferred electroplating metal isnickel, however, copper and tin may also be used. Electroplating is anefficient method of achieving the desired thickness of the seal, whichis in the order of tens to hundreds of microns. Methods other thanelectroplating may be used to deposit metal for the sealant wallincluding electroless plating, evaporation, and sputtering, for example.

Step 910 may also include electroplating a backplane adhesion layer tothe sealant wall, wherein the backplane adhesion layer comprises asolder, such as PbSn, InSb, SnBi, or other solders capable of beingplated. In some embodiments, the backplane adhesion layer comprisesmultiple layers, such as a wettable metal layer and a solder layer.

In a step 912, the mask, metal layers, and insulator are removed atareas not covered by the electroplated metal by a method such as a wetchemical etch. Alternatively, the metal layers 804 and insulator 802 mayeach be removed in separate steps. The metal seed layer may be removedby ion milling, for example. In the embodiment wherein the metal seedlayer is deposited on top of the mask 806, the metal seed layer isremoved with the mask 806.

In step 914, prior to joining the backplane to the sealant wall, thesacrificial layer present in the interferometric modulator array isetched, using XeF₂ gas for example, to release the membranes orconductive members of the interferometric modulator elements of thearray. Retaining the sacrificial layer through the seal formationprocess 900 may be preferable to protect the array from damage that canoccur during any steps of the seal forming process 900. However, thesacrificial molybdenum can be removed any other times during thefabrication process.

In a step 916, after the sacrificial layer is removed in step 914, thebackplane is joined to the sealant wall to form the package structure.In one embodiment the backplane is joined to the sealant wall by solderto create a hermetic joint. The solder can be deposited on the sealantwall as discussed in reference to step 910, can be applied after steps912 and/or 914, or can be applied to the backplane. The solderpreferably melts and flows at a temperature less than about 250° C. soas to avoid heat damage to the interferometric modulator array 76.

The backplane may have one or more backplane adhesion layers depositedthereon for adhesion to the sealant wall, or wettable metal and/orsolder deposited on the sealant wall. In one embodiment, the backplanecomprises an adhesion layer, such as a thin metal like Cr or Ti, orother material configured to withstand soldering temperatures, andsolder is deposited on the adhesion layer for attachment of thebackplane to the sealant wall. In one embodiment, the backplanecomprises a metal that is readily solderable. In another embodiment, thebackplane comprises a thin film of metal or a path of metal that issoldered to the sealant wall or joined using a seamseal. The formationof the seal 78 reduces the complexity of the packaging process by makingthe formation of the seal part of the array process.

While the above detailed description has shown, described, and pointedout novel features of the invention as applied to various embodiments,it will be understood that various omissions, substitutions, and changesin the form and details of the device or process illustrated may be madeby those skilled in the art without departing from the spirit of theinvention. Additionally, the steps which are described and illustratedherein is not limited to the exact sequence of acts described, nor is itnecessarily limited to the practice of all of the acts set forth. Othersequences of events or acts, or less than all of the events, orsimultaneous occurrence of the events, may be utilized in practicing theembodiments of the invention. As will be recognized, the presentinvention may be embodied within a form that does not provide all of thefeatures and benefits set forth herein, as some features may be used orpracticed separately from others.

1. An electronic device comprising a micro-electromechanical systems(MEMS) device, a metal layer, a mask, a metal seal layer, and abackplane prepared by a process comprising: providing a MEMS device on asubstrate; depositing a metal layer on the substrate; forming a maskwith one or more perimeter cavities over the metal layer; depositing oneor more metal seal layers in the one or more perimeter cavities, therebyforming a seal proximate to the perimeter cavities of the MEMS device;and joining a backplane to the seal.
 2. The electronic device of claim1, wherein the MEMS device comprises an interferometric modulator. 3.The electronic device of claim 1 further prepared by a processcomprising removing the mask, and the metal layer, at areas not coveredby the one or more metal seal layers, thereby forming a sealant wall. 4.The electronic device of claim 3 further prepared by a processcomprising removing a sacrificial layer from the MEMS device.
 5. Theelectronic device of claim 1 further prepared by a process comprisingapplying a desiccant between the MEMS device and the backplane.
 6. Theelectronic device of claim 5, wherein the desiccant is selected from thegroup consisting of zeolites, molecular sieves, surface adsorbents, bulkadsorbents and chemical reactants.
 7. The electronic device of claim 1,wherein the desiccant comprises a powder.
 8. The electronic device ofclaim 1, wherein the seal proximate to the perimeter of the MEMS devicecomprises a non-hermetic seal.
 9. The electronic device of claim 8,wherein the non-hermetic seal is selected from the group consisting ofconventional epoxy-based adhesive, polyisobutylene, butyl rubber,o-rings, polyurethane, thin film metal weld, liquid spin-on glass,solder, polymers and plastic.
 10. The electronic device of claim 1,wherein the seal proximate to the perimeter of the MEMS device comprisesa substantially hermetic seal.
 11. The electronic device of claim 10,wherein the substantially hermetic seal comprises metal.
 12. Theelectronic device of claim 1 further prepared by depositing an insulatorlayer on the substrate before depositing the metal layer.
 13. Theelectronic device of claim 12, wherein the depositing the insulatorlayer comprises depositing the insulator layer to contact the substrate.14. The electronic device of claim 12, wherein the depositing theinsulator layer comprises depositing the insulator layer on at least onelayer in contact with the substrate.
 15. The electronic device of claim14, wherein the at least one layer comprises a conductive material.